The present invention relates to the field of current measurement. More specifically, the present invention relates to apparatus and a method for measuring load current in an alternating current (AC) inverter drive system.
It is well known in the art that a measurement of the load current can provide means for protecting the drive system during an overload situation. It is also known that knowledge of the load current can be used as a means of measuring the output of the drive system.
In one technique, load currents in an AC drive system are measured by fitting isolating current transducers to a motor output. However, drive systems incorporating such devices are expensive.
It is also know to measure load currents by sampling the motor currents in the direct current (DC) link of the inverter. However, drive systems employing this sampling technique are sensitive to noise and measurement black spots during the pulse-width modulation (PWM) cycle when data is not available or unreliable. This problem is particularly severe at high switching frequencies.
Furthermore, the mean DC link current in the open loop AC inverter drive system can be measured by integrating over an integral number of PWM cycles. This method reduces the effects of noise on the measurement reliability. However, while this method can be used to measure the real component of the load current, the imaginary component of the current is still unknown and must be estimated. Thus any subsequent calculation of the total load current is only as accurate as the estimation of the imaginary component.
FIG. 1 is a circuit diagram of a known voltage source DC link inverter 100 which measures the load current by sampling the current flowing through the DC link 2. This type of inverter is often used in open loop AC drive systems. The DC link comprises a positive DC input 3 and a negative DC input 4, which are both in electrical connection with capacitor 6. The current is measured through the negative DC input 4 with current measurement means 12. In this example the current measurement means is a shunt. The use and operation of shunts is well known in the art of electronics. The inverter 10 is a three phase inverter comprising a first switch 14, a second switch 16, and a third switch 18. Each of the first, second, and third switches includes a respective pair of transistors, or other suitable electronic switching element: 14a and 14b, 16a and 16b, 18a and 18b; a respective pair of diodes: 14c and 14d, 16c and 16d, 18c and 18d. Each of the first, second, and third switches also comprises a respective output: 7, 8, and 9. The outputs are electrically connected to a motor (not shown).
The shunt 12 functions to measure the DC link current (IDC) into inverter 10 via the negative DC input 4. The instantaneous value of IDC can be used to provide protection to the device in situations such as drive system output short circuit. The real component of the current (Ireal) may be calculated from the mean value of IDC and used to facilitate control of the motor. However, to calculate the actual magnitude of the load current the imaginary component of the current must be estimated.
Estimation of the imaginary component of the load current can lead to errors in the calculation of the load current magnitude. These errors are worse at certain frequencies and drive system loads. Furthermore, the calculation of the real component of the current involves a division by the modulation depth of the PWM. This quantity is unacceptably inaccurate at low modulation depths and high switching frequencies.
It is an object of the present invention to provide apparatus and a method for measuring the load current in which an accurate calculation of the load current is obtained without the need for estimating the imaginary component.
According to the present invention there is provided apparatus for measuring current flowing from an inverter to a load, comprising a first current measuring means and a second current measuring means, said first current measuring means and said second current measuring means being arranged such that said first current measuring means measures a first current flowing through an inverter and said second current measuring means measures a second current flowing through said inverter.
According to an aspect of the present invention, said first current measuring means is coupled to a plurality of electronic switches forming part of said inverter, and said second current measuring means is coupled to a plurality of diodes forming part of said inverter, said first current measuring means and said second current measuring means being arranged such that said first current measuring means measures a first current flowing through said plurality of electronic switches and said second current measuring means measures a second current flowing through said plurality of diodes.
According to an alternative aspect of the present invention, said first current measuring means is coupled to a negative DC input forming part of a DC link to said inverter, and said second current measuring means is coupled to plurality of electronic switches forming part of said inverter, said first current measuring means and said second current measuring means being arranged such that said first current measuring means measures a first current flowing through said negative DC input and said second current measuring means measures a second current flowing through said plurality of electronic switches.
According to yet another alternative aspect of the present invention, said first current measuring means is coupled to a negative DC input forming part of a DC link to said inverter, and said second current measuring means is coupled to plurality of diodes forming part of said inverter, said first current measuring means and said second current measuring means being arranged such that said first current measuring means measures a first current flowing through said negative DC input and said second current measuring means measures a second current flowing through said plurality of diodes.
According to a further aspect of the present invention, said apparatus further comprises processing means coupled to said first and said second current measuring means, said processing means being arranged to calculate said load current based upon said first and second currents.
Said processing means may be further arranged to calculate the real component of the said load current based upon said first and second currents.
The current measuring means may be shunts. The electronic switches may be transistors.
Said apparatus may be included in an AC drive system. The drive system may be an open loop system. The drive system may use pulse width modulation. The pulse width modulation may be space vector modulation.
According to the present invention there is provided a method of measuring current flowing from an inverter to a load, said method comprising the steps of: measuring a first current flowing through said inverter, measuring a second current flowing through said inverter, and calculating said load current based upon said first and second currents.
According to a method aspect of the present invention, said inverter comprises a plurality of electronic switches and a plurality of diodes, said step of measuring a first current is a measurement of a current flowing through said plurality of electronic switches, and said step of measuring a second current is a measurement of a current flowing through said plurality of diodes.
According to an alternative method aspect of the present invention, said inverter comprises a plurality of diodes and is coupled to a negative DC input of a DC link, and said step of measuring a first current is a measurement a current flowing through said negative DC input, and said step of measuring a second current is a measurement of a current flowing through said plurality of diodes.
According to yet a further alternative method aspect of the present invention said inverter comprises a plurality of electronic switches and is coupled to a negative DC input of a DC link, and said step of measuring a first current is a measurement a current flowing through said negative DC input, and said step of measuring a second current is a measurement of a current flowing through said plurality of electronic switches.
Said method may include a further step of calculating a real component of said load current based upon said first and second currents.
Said method may include yet a further step of calculating an imaginary component of said load current based upon said load current and said real component of said load current.
Advantageously, the present invention provides for all measurements to be DC quantities which are integrated over an integral number of PWM cycles to cancel any adverse effects due to switching frequency harmonic noise.
Advantageously, the circulating currents are measured directly thus eliminating inaccuracies in the load current measurement due to divisions by small numbers when operating at low modulations depths.
Furthermore, the transistor current measurement may also be used for drive system protection.
While the principle advantages and features of the invention have been described above, a greater understanding and appreciation of the invention may be obtained by referring to the drawings and detailed description of the preferred embodiments, presented by way of example only, in which;